Building git commit objects with git hash-object?

Question

How can i manually build git commit objects using git hash-object? I now it works with blobs, and its documentation says it can build different objects by using -t but how do you build a commit with that?

Answer 1

Here's a complete and working example of a script that creates a commit object without ever running git commit:

mkdir project
cd project
git init

hash=`echo -n "" | git hash-object -w --stdin`

tree=`echo -e "100644 blob ${hash}\temptyfile" | git mktree`

commit=`echo -e "yourname\[email protected]\n2013 12:20:15 +0200\ncommittername\[email protected]\n2013 10:13:15 +0200" | git commit-tree ${tree}`

git update-ref refs/heads/master ${commit}

To verify that the script created a commit containing an empty file, run:

git checkout --
git log --oneline
#2abbdc2 yourname [email protected] 2013 12:20:15 +0200 committername [email protected]

Edit: fixed and improved

Answer 2

For the benefit of the reader:

The accepted answer from Arialdo Martini is fully correct and explains how to create an empty git repo with proper plumbing commands. Please note that his variant works for bare repositories, too (you can create emptyfile in the git-dir with no bad sideeffects).

This answer here sums it up into a script with a minor tweak: The contents of the file is taken from "stdin" and the file can be placed into a sub-directory of the worktree.

Script git-init-with-file-from-stdin.sh new-git-workdir filename 'commit message':

#!/bin/bash

mkdir "$1" &&
cd "$1" &&
git init &&

dir="$(dirname "$2")" &&
name="$(basename "${2:-dummyfile}")" &&

obid="$(git hash-object -w --stdin)" &&
treeid="$(git mktree < <(printf '100644 blob %q\t%q\n' "$obid" "$name"))" &&

if [ . = "$dir" ]; then git read-tree -i "$treeid";
else git read-tree --prefix="$dir/" -i "$treeid"; fi &&

git commit -m "${3:-automatic commit}" &&
git reset --hard

Explained for the call

    ./git-init-with-file-from-stdin.sh newgitdir path/to/file <<< "hellOw W0rld"

• mkdir "$1", cd "$1", git init creates the new git worktree and initiailizes it. It is named after the first argument (here newgitdir), which is assumed to name a nonexisting directory in an existing path.

• dir="$(dirname "$2")", name="$(basename "${2:-dummyfile}")" extracts the path and the name part of the second argument, which defines the wanted filename. The path is relative to the created git-workdir. Note that this argument must not start with a /, else the command fails. Here dir=path/to and name=file. If left away, the new file is called "dummyfile" in the top of the git-workdir.

• obid="$(git hash-object -w --stdin)" then stores the object with the information read from stdin into the git repo and stores the SHA (object id) of the new object in the variable obid. In the example, the contents is hellOw W0rld followed by an NL.

• git mktree < <(printf '100644 blob %q\t%q\n' "$obid" "$name") is nearly the same as printf '100644 blob %q\t%q\n' "$obid" "$name" | git mktree and creates a proper git-tree. 100644 is the usual file mode. If you want to create executables, you need 100755 here.

• treeid="$(...)" then assigns this to the given variable treeid

• if [ . = "$dir" ]; then git read-tree -i "$treeid"; this reads this newly created tree into the git staging area for later commit. However this is for the case, that the file shall be directly in the git-workdir.

• else git read-tree --prefix="$dir/" -i "$treeid"; fi is the same for the case, when you want to put it into a subdirectory of the git-workdir. The nice thing is, that git creates all intermediate directory nodes for you, automatically. (Sadly --prefix="./" produces an error.)

• git commit -m "${3:-automatic commit}" then uses the well-known commit to create the commit

• git reset --hard then syncs the git-worktree with the latest commit.

Changes for bare variant:

• Everything works similar, except git commit (this needs a workdir) and git reset (you do not need it).

• replace

  git init &&
  

  with

  git init --bare &&
  

  also replace

  git commit -m "${3:-automatic commit}" &&
  git reset --hard
  

  with a similar sequence as seen in the accepted answer:

  commitid="$(git commit-tree "$(git write-tree)" <<< "${3:-automatic commit}")" &&
  git update-ref "refs/heads/master" "$commitid"
  

Remarks:

• See also https://stackoverflow.com/a/25556917

• This here needs bash, which means, it works for Windows 10, too